Warm air furnaces are frequently used in homes and office buildings to heat intake air received through return ducts and distribute heated air through warm air supply ducts. Such furnaces typically include a circulation blower or fan that directs cold air from the return ducts across a heat exchanger having metal surfaces that act to heat the air to an elevated temperature. A gas burner is used for heating the metal surfaces of the heat exchanger. The air heated by the heat exchanger can be discharged into the supply ducts via the circulation blower or fan, which produces a positive airflow within the ducts. In some designs, a separate combustion blower can be used to remove exhaust gasses resulting from the combustion process through an exhaust vent.
In a conventional warm air furnace system, gas valves are typically used to regulate gas pressure supplied to the burner unit at specific limits established by the manufacturer and/or by industry standard. Such gas valves can be used, for example, to establish an upper gas flow limit to prevent over-combustion or fuel-rich combustion within the appliance, or to establish a lower limit to prevent combustion when the supply of gas is insufficient to permit proper operation of the appliance. In some cases, the gas valve regulates gas pressure independent of the combustion blower. This may permit the combustion blower to be overdriven to overcome a blocked vent or to compensate for pressure drops due to long vent lengths without exceeding the maximum gas firing rate of the furnace.
In some designs, the gas valve may be used to modulate the gas firing rate within a particular range in order to vary the amount of heating provided by the appliance. Modulation of the gas firing rate may be accomplished, for example, via pneumatic signals received from the heat exchanger, or from electrical signals received from a controller tasked to control the gas valve. While such techniques are generally capable of modulating the gas firing rate, such modulation is usually accomplished via control signals that are independent from the control of the combustion air flow. In some two-stage furnaces, for example, the gas valve may output gas pressure at two different firing rates based on control signals that are independent of the actual combustion air flow produced by the combustion blower. Since the gas control is usually separate from the combustion air control, the delivery of a constant gas/air mixture to the burner unit may be difficult or infeasible over the entire range of firing rate.
To overcome this problem, attempts to link the speed of the combustion blower to the gas firing rate have been made, but with limited efficacy. In one such solution, for example, the fan shaft of the combustion blower is used as a pump to create an air signal that can be used by the gas valve to modulate gas pressure supplied to the burner unit. Such air signal, however, is proportional to the fan shaft speed and not the actual combustion air flow, which can result in an incorrect gas/air ratio should the vent or heat exchanger become partially or fully obstructed. In some cases, such system may result in a call for more gas than is actually required, reducing the efficiency of the combustion process.
In another common modulating technique in which zero-governing gas pressure regulators and pre-mix burners are used to completely mix gas and air prior to delivery to the burner unit, an unamplified (i.e. 1:1 pressure ratio) pressure signal is sometimes used to modulate the gas valve. Such solutions, while useful in gas-fired boilers and water heaters, are often not acceptable in warm air furnaces where in-shot burners are used and positive gas pressures are required.
Other factors such as complexity and energy usage may also reduce the efficiency of the gas-fired appliance in some cases. In some conventional multi-stage furnaces, for example, the use of additional wires for driving additional actuators on the gas valve for each firing rate beyond single-stage may require more power to operate, and are often more difficult to install and control. Depending on the type of modulating actuators employed, hysteresis caused by the actuator's armature traveling through its range of motion may also cause inaccuracies in the gas flow output during transitions in firing rate.